Referring first to FIG. 1, a feed mechanism equipped with a threaded member 4 is schematically shown. This mechanism further includes an electric motor 1, a bearing 2, and a coupling gearing 3. The threaded member 4 is mounted between the motor 1 and the bearing 2, and is rotated by the motor in forward or reverse direction via the gearing 3. A guide bar 5 is mounted parallel to the threaded member 4, and a moving member 6 is mounted over the bar 5 such that the member 6 can make a sliding motion along the bar. A leaf spring 7 depends from the bottom of the moving member 6 and has a pointed portion 8 at its end. This portion 8 is fitted into a helical groove 9 formed in the periphery of the threaded portion 4 so that the portion 8 may slide within the groove. Thus, the moving member 6 can be reciprocated within a certain region while guided by the guide bar 5 by rotating the threaded member 4 in forward or reverse direction.
When it is desired to drive the moving member 6 intermittently in given steps, the motor 1 is energized intermittently to rotate the threaded member 4 in a stepwise way, whereby the moving member 6 is advanced in a given direction.
The conventional intermittent feed member, i.e., the threaded member 4, of the feed mechanism is so designed that the helical groove 9 has the same lead angle .theta. over the whole length, as shown in FIG. 2, and that if the groove 9 is developed onto a plane, it forms a straight line. Therefore, if it is desired to stop the movement of the moving member 6 in stepwise manner at eight angular positions 0.degree., 45.degree., 90.degree., 135.degree., 180.degree., 225.degree., 270.degree., and 315.degree., the unevenness of the helix of the threaded member 4 or error of control over the rotation displaces the moving member 6 from its proper position proportionately, so that the member 6 may not stop at correct positions, thus imparing the reliability. Further, since the error of control over the rotation tends to produce an error in the quantity of feed as described above, an accurate and hence expensive motor must be used.